The present invention relates to structures such as a radome which transmit and reflect high frequency radio signals, and in particular to structures incorporating multi-layer slotted screens.
The basic function of a radome is to protect an antenna from environmental factors. The radome must typically protect an antenna while simultaneously the radome must not interfere with the electrical operation of a radar system. Radomes can range from simple plastic bubbles and air inflated enclosures for stationary systems, to structures which exhibit high structural strength and abrasion resistance in applications such as airborne radar systems. In the case of airborne systems, weight and structural strength become important factors and solutions to these requirements often result in degradation of the radar systems performance.
Conversely, a properly designed radome can enhance operation of a radar antenna in some respects. For example, a radome which exhibits a highly selective pass band and a high Q for the pass band will allow the radar system to operate effectively within its own operating bandwidth while simultaneously reflecting or rejecting other signals which lie outside the pass band of the system. This can be a very important factor in military applications where externally produced jamming signals and other friendly signals can degrade system performance.
One proposed solution for providing a radome structure which exhibits high transmission efficiency and a desirable pass band characteristic is a metallic screen having a multiplicity of slots therethrough. The screen is covered with, and the slots are filled with, a suitable dielectric material to fully close the structure and to control the electrical characteristics of slot configuration for transmission efficiency. A rigorous analysis of this structure is presented in a paper titled, "Transmission Through a Conducting Screen Perforated Periodically With Apertures" by Chao-Chun Chen published in IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-18, 9, September 1970, page 627. This structure, while it exhibits some advantageous bandpass characteristics, also exhibits the undesirable characteristic of producing a frequency shift in a transmitted signal as a function of its angle of incidence on the radome structure.
In a subsequently proposed structure, the thin conductive screen analyzed by Chen was replaced with a thick conductive screen again perforated with periodically spaced apertures. The screen is enclosed in a dielectric sandwich with the dielectrics selected to modify the transmission characteristics of the structure. This structure is rigorously discussed and analyzed in the paper titled, "Some Effects of Dielectric Loading on Periodic Slot Arrays" by R. G. Lubbers, and B. A. Munk published in IEEE Transactions on Antennas and Propagation, Vol. AP-26, No. 4, July 1978, page 536. The thick screen is also analyzed in the paper entitled, "On The Theory And Solar Application of Inductive Grids", by R. C. McPhedran and D. Matystre published in Applied Physics, Vol. 14, January 1977, page 1. This structure exhibits an improved bandpass characteristic, and substantially reduced frequency shift degradation of a transmitted signal as a function of incident angle. However, due to the required thickness of the conductive screen, the structure is relatively heavy for airborne applications and the screen thickness presents significant manufacturing difficulties and expense.
It is therefore a feature of the present invention to provide a radome structure that exhibits a high Q bandpass characteristic which is light in weight, structurally sound, and economical to manufacture.